AcousticSpace_First Acoustic Analysis

As I already said in previous posts,
the hangar renovation has to gain acoustic feautures. Therefore i set a first series of analysis whose results are the following.

1_purpose of the analisys

In the clients' intention, the hall has to be a conference hall or a sort of cinema. The analysis has a double acoustic purpose. 

At first, it essential that the reverberation time loweres strongly. It is difficoult to quantify how much should it be lowered because we do not know its actual value (in order to make a good analisys we should need a session of in-situ measurements indeed).

On the other hand, other acoustic paramethers shall be good or at least acceptable (see AcAna_paramethers to take in account post). 

2_limits of the analisys

Ramsete 2.5 is the software i used. This pyramid-tracing software offers a pretty good simulation, and its prediction can be excellent. But the origin data have to be good as well ("garbage in, garbage out").In our case we have two strong limits on the analisys, both of them due to a lack of origin data.

At first: we have no in-situ measurements of the current situation in the hangar. These measurements have a cost, which at present is not necessary and therefore not affordable by clients.

Moreover, Barrisol's acoustic data are not exactly what we need.

Indeed, Barrisol offers a series of acoustic values for simulation like this one. These values are not a-priori, because they are the result of measurements on 1:1 models. The further our model is far from those conditions, the worse the result will be given by simulations (see next paragraph).

Without these data are results not good? No, but they have to be interpretated as a general trend for the project's effect, not as real predictions.

3_about Barrisol

Barrisol is a blanket which can or cannot have acoustic features. This depends on what kind of barrisol we're buying Acoustic or Not. Moreover, its acoustic behaviour is strongly influenced by "what's behind it". Thus because it is a blanket, it has neither mass nor volume. It just defines a space but has no capability to absorbe energy from the sound wawe: it just transmit (and reflect) it, and therefore the most important element in the absorption process is the one receiving the amount of energy transmitted (a certain amount of energy is obviously trasmitted from the air to the blanket, whose vibration make it lose).

In picture we are looking at different absorbtion coefficient for the barrisol. We have to say now that, since project's budget is strictly limited, we cannot efford barrisol_acoustic. Moreover, we clearly see how close are the values for Barrisol Acoustic/not, in lack of rockwool case.

What influences the real absorption is therefore 1. material behind blanket (together with the pobbibility for the sound wave to reach this material: holes' percentage. Without holes this process works the same, but a far less) 2. distance between material and blanket. In this picture we see how do absorbed frequencies range variates as a result of the distance itself. This phonomenon exist on Not Acoustic Barrisol as weel, it it stronger indeed.

Moreover in our case The distance is 1. not constant and 2.in the order of bigness of the one of m, while in the second graph it is cm. Results are therefore strongly influenced by the fact that we are not using these data in a conventional way (even if the teorethical effect should be better for us, since the distance is not constant and therefore frequencies range variates point by point). 

At present i've no better Barrisol data (i had asked. for....).

4_Experiment i set up

I set up a series of 7 simulations:

0_ actual condition (simulated! i tried and assigm materials in order to obtain a result similar to my perception of the actual condition)

A_drunk space                                               B_quadratic space

(i assigned barrisol without rockwool values to blakets in the model_for me b + rockwool values are not acceptable at all)

A_with acoustic end                                            B_with acoustic end

(halls are closed by an acoustic barrisol blanket+ rockwool_this is the usual situation for a conference hall)

A_FAKE                                               B_FAKE

Since we cannot try the effective barrisol's impact on the project, i've tested the form only: realising fake models which has same form, but which i assigned hangar material (steel sheets) to. In my opinion, it is the form indeed (and not the material) which has the power to lower the reverberation time, since it create a new volume, smaller by far.

Simulations i've been making have a good level of precision, while they consider the first 100 reflections only. This condition is a good approximation.

NB:I've taken measurements in the inside-blankets volume only, at the height of the hear. Results are therefore referring to the acoustic space only (ramsete interpolates values: outer one are not good, in pictures)

I'm publishing now on results of these analisys.

5a_RESULTS: reverberation Time 

We clearly notice what we expected: it's the form only, not the absorbtion power of each material (which is extremely poor in barrisol only case as well),lowering the Trev. The loss of sound tail is on the order of 40%, and this simulations are made in case of empty hall, therefore another contribution (around 15%) will by given by the audience. As said, we cannot consider numeric values, because of origin data but we mantain that the lowering given by either blanket or blks+acoustic-end is too small for a conference hall, whose Trev is close to 1s.

5b_RESULTS: hall's impulse response

Ramsete is not able to evaluate Initial Time Delay Gap, which is probably the most important paramether for the quality of the listening (in my recent posts, it quantifies what i callled haas effect).

In this image we see that the hall response is good in every case. Since the variation is close to the extreme one (6db) and material's values are not totally reliable, it should be necessary an electronic system  to strengthen the sound. I remember that we are in case of void hall: with people listening this need shall be more effective.

Lateral Envelopement, an important paramether giving the spatial impression of the audience, is everywhere acceptable, but not outstanding.

We mantain that the hall has a good acoustic response.

5c_RESULTS: hall's proprieties

These images show results for important paramethers which discriminates the use of an hall (music-speech). We have to say now that these values are extremely influenced by the bad origin data! and so these are nothing but an advice.

Values collected for Clarity in the forst 50ms show that the hall seem not to be this good for speech...

...while the values collected from the C80 show a good capability for music listening (Situation A_acoustic exceeds expectations).

Thus confirmed by Definition Values as weel.

In the end: the "drunk" solutions offer better performances for the music listening (because of its more irregular form, reflecting soung waves more randomly); both of them does not have at present features to become a good conference hall. Thus beacuse the absorbtion mechanism is linked to form and not material caratheristics. In order to make the hall become a conference hall, it shal be necessary

1)further measurements

2)further data

3)other system of sound absorption (when we listen to speech, we need to identify better -more clarity, more definition- each word, and each word has to decay soon, in order to make a speech comprehensible.While we listen to music, on the other end, most of the armony perception is given by the melting of different sounds: therefore we need an higher persistence of each sound, and a smaller clarity and definition).